Abstract: A tractor trailer having an air brake system includes a reservoir of pressurized air for holding its brakes applied when parked. A fail safe locking device is provided, which is operable to lock the brake actuator rod in the fixed position in which the brakes are applied upon loss of air reservoir pressure. The locking device includes canting plates and an air cylinder. The air pressure from the reservoir overcomes a biasing mechanism in the locking device to shift the canting plates to a normal rod release position. Upon loss of pressure, the biasing mechanism shifts the air cylinder to tilt the canting plates and lock the brake actuator rod in a fixed position. The actuator rod is actuated longitudinally to rotate a brake crank arm and crank rod to apply the vehicle brakes. The crank arm interfits into a clevis extension on the actuator rod and is connected thereto by a rigid connector.